This invention relates generally to methods and apparatus for cooling a superconducting magnet, and specifically to cooling a magnet used in a magnetic resonance imaging (MRI) system.
There are various magnetic imaging systems which utilize superconducting magnets. One example of an imaging system is a magnetic resonance imaging (MRI) system. MRI systems are used to image a portion of a patient's body.
Superconducting MRI systems typically utilize one superconducting magnet, often with multiple coils. An imaging volume is provided inside the magnet. A person or material is placed into an imaging volume and an image or signal is detected and then processed by a processor, such as a computer.
The majority of existing superconducting MRI magnets are made of a niobium-titanium material which is cooled to a temperature of 4.2 K with a cryostat. A typical superconducting magnet cryostat includes a liquid helium vessel, one or two thermal shields and a vacuum vessel. The thermal shields intercept radiation from the ambient atmosphere to the helium vessel. The heat load from this radiation is balanced with a refrigerator, such as a cryocooler, which provides cooling to the cryostat.
However, in addition to the heat from ambient atmosphere, there may be additional sources of heat. For example, when the cryostat components are exposed to magnetic fields, for instance the fringe field from an MRI gradient coil, eddy currents are produced. These eddy currents generate joule heating which increases the heat load to be removed by the refrigerator. Moreover, the magnetic fields produced by these eddy currents in the image volume generally have adverse impact on the imaging quality of the system. Prior attempts to address this problem consisted of reducing the AC field strength in the area of the superconducting coils and cryogenic components. However, this typically results in an adverse impact on the system performance.